MOVEMENTS OF THE EYEBALL. 



809 



not at right angles with the axis of the eye, but is inclined from the nasal side slightly 

 backward. The line which serves as the axis of rotation for these muscles forms an angle 

 of about seventy degrees with the axis of the globe ; and, as a consequence of this arrange- 

 ment, their action is not so simple as that of the internal and external recti. The inser- 

 tion of the superior rectus is such, that, when it contracts, the pupil is directed upward 

 and inward, the inferior rectus directing the pupil downward and inward. 



The above represents the simple, isolated action of each pair of recti muscles ; but it 

 is easy to see how, without necessarily involving the action of the oblique muscles, the 

 globe may be made to perform an immense variety of rotations, and the line of vision may 

 be turned in nearly every direction, by the action of the recti muscles alone. 



Action of the Oblique Muscles. Although there has been considerable discussion con- 

 cerning the exact mode of action of the oblique muscles, their mechanism may now be 

 regarded as pretty well settled, at least as regards the human subject. In the first place, 

 it is sufficient for all practical purposes, to assume that the superior and the inferior 

 oblique muscles act as direct antagonists to each other. The next point to determine is 

 the direction of the axis of rotation of the globe with reference to the action of these 

 muscles. The most exact, recent measurements show that this axis is horizontal and 

 that it has an oblique direction from before backward and from without inward. The 

 angle formed by the axis of rotation of the oblique muscles with the axis of the globe is 

 thirty-five degrees; and the angle be- 

 tween the axis of the oblique muscles 

 and the axis of the superior and inferior 

 recti muscles is seventy-five degrees. 



Given the direction of the axis of 

 rotation and the direction of the supe- 

 rior oblique muscle, it is easy to under- 

 stand the effects of its contraction. As 

 this muscle, passing obliquely backward 

 and forward over the globe, acts from 

 the pulley near the inner angle of the 

 eye to its insertion just behind the an- 

 terior half of the globe on its external 

 and superior surface (7, Fig. 254), it 

 must rotate the globe so as to direct the 

 pupil downward and outward. 



The inferior oblique, passing outward 

 and slightly backward under the globe, 

 acts from its origin at the margin of the 

 orbit near the inner angle of the eye to 

 its insertion, which is just below the in- 

 sertion of the superior oblique. This 

 muscle rotates the globe so as to direct 

 the pupil upward and outward. 



The action of the oblique muscles 

 seems to be specially connected with 

 the movements of torsion of the globe. 

 It is necessary to distinct, single vision 

 with both eyes, that the images should 

 be formed upon exactly corresponding points on the retina, and that they should bear, 

 for the two eyes, corresponding relations to the perpendicular. Thus it is that, when the 

 head is inclined to one side, the eyes are twisted upon an oblique, antero-posterior axis ; 

 as can be readily observed if we watch little spots upon the iris during these movements. 



next. 



FIG. 255. Diagram illustrating the action of the muscles of 



the eyeball. (Fick.) 

 The dark lines represent the muscles of the eyeball, and the 



dotted lines, the axis of the superior and the inferior rectus 



and the axis of the oblique muscles. 



